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Quantum cryptography studies the unconditional information security againstan all-powerful eavesdropper in secret key distillation. However the assumptionof an omnipotent eavesdropper is too strict for some realistic implementations.In this paper we study the realistic application model of secret keydistillation over satellite-to-satellite free space channel in which we imposea reasonable restriction on the eavesdropper by setting an exclusion zonearound the legitimate receiver as a defense strategy. We first study the casewhere the eavesdropper's aperture size is unlimited so her power is onlyrestricted by the exclusion zone. Then we limit Eve's aperture to a finite sizeand study the straightforward case when her aperture is in the same plane ofBob, investigating how an exclusion zone can help improve security.Correspondingly, we determine the secret key rate lower bounds as well as upperbounds. Further more, we also apply our results on specific discrete variable(DV) and continuous variable (CV) protocols for comparison. We show that byputting reasonable restrictions on the eavesdropper through the realisticassumptions of an inaccessible exclusion zone, we can increase the key rate incomparison to those without and do so with relatively lower transmissionfrequency. We conclude that this model is suitable for extended analysis inmany light gathering scenarios and for different carrier wavelengths.

Secret key distillation over realistic satellite-to-satellite free-space channel: exclusion zone analysis